The present invention is related to an environmental control system. More particularly, the present invention is related to the control and operation of a dual duct device such as a dual duct VAV box.
Environment control networks or facility management systems are employed in office buildings, manufacturing facilities, and the like, for controlling the internal environment of the facility. The environment control network may be employed to control temperature, fluid flow, humidity, lighting, boilers, chillers, or security in the internal environment. A particular type of environmental control system is a dual duct HVAC system configured to control temperature and air flow in an internal environment. The dual duct HVAC system is generally comprised of dual duct controlled air units (e.g., variable air volume (VAV) boxes or unitary devices (UNT)) located throughout a facility. The VAV boxes provide environmentally controlled air to the internal environment. The controlled air is provided at a particular temperature or humidity so that a comfortable internal environment is established.
The dual duct VAV boxes or other unitary devices are coupled to an air source which separately supplies hot air and cold air to the VAV box via duct work. The hot air is provided by a hot air duct, and the cold air is provided by a cold air duct. VAV boxes and unitary devices may include a fan or other device for blowing the hot and cold air.
VAV boxes and unitary devices provide the hot and cold air through dampers (e.g., a hot air damper and a cold air damper). The dampers regulate the amount of the hot and cold air provided to the internal environment. The dampers are coupled to actuators which position the dampers so appropriate mixed air flow (the combination of the hot air flow and cold air flow) measured in cubic feet per minute (CFM) is provided to the internal environment. The dual duct VAV box provides the mixed air flow to the hallway, room, office, portion of a building or other internal environment.
A digital controller is generally associated with each dual duct VAV box or other device containing the hot air damper, the cold air damper and their associated actuators. The controller receives information related to the air flow and temperature in the internal environment and generates cold air flow setpoints and hot air flow setpoints. The controller appropriately positions the actuators in accordance with the setpoints so that the appropriate mixed air flow is provided to the internal environment. The controller may include sophisticated feedback mechanisms such as proportional integral derivative (PID) control algorithms. Sophisticated feedback mechanisms allow the actuators to be positioned more precisely.
Heretofore, the controller computed the set points for the hot and cold air flow in response to the measured room temperature. Sophisticated dual duct control algorithms employed within the controller continually reset and recalculate the hot and cold air flow setpoints in response to the measured room temperature. These conventional dual duct control algorithms are prone to hunting as the actuators are repositioned in accordance with newly calculated hot and cold air flow setpoints. Hunting often causes noise and air flow changes which can annoy the occupant. Hunting also excessively wears the actuators associated with the hot and cold air dampers.
Thus, there is a need for a dual duct control system which is not prone to hunting. Further, there is a need for a dual duct control system which accounts for the present cold air flow and hot air flow setpoints when calculating new setpoints. Further still, there is a need for a dual duct control system which measures the temperature of the mixed air and the flow of the mixed air and generates new setpoints in response to those measurements.